Class MissingArgEval

Represents the (intermediate) evaluated result of a missing function argument. In most cases
this can be translated into BlankEval but there are some notable exceptions. Functions
COUNT and COUNTA do count their missing args. Note - the differences between
MissingArgEval and BlankEval have not been investigated fully, so the POI
evaluator may need to be updated to account for these as they are found.